A conventional two-cycle engine mounted on portable work machine sometimes has an arrangement in which a crankcase is divided into halves. According to such a halved structure, when a crankshaft is supported on both sides of the crankcase with a crank portion interposed therebetween, the crankcase is divided into two half bodies, specifically, one half body for supporting one end of the crankcase and the other half body for supporting the other end of the crankcase (for example, see Patent Documents 1 and 2).
In such a halved structure, the one half body is provided with a step that externally holds a shoulder of an outer ring of a bearing to prevent the bearing that supports the crankshaft from dropping to the outside. At the same time, a shoulder of an inner ring of the bearing is internally held by a step of the crankshaft to prevent the bearing from moving inwardly.
On the other hand, unlike the one half body, the other half body is not provided with a step due to manufacturing restrictions. A shoulder of an outer ring of the other bearing is held by an externally-disposed holder such as an E-ring. An inner ring of the other bearing is held in the same manner as in the one half body.
The step of the half body holds the shoulder of the outer ring of the bearing more securely than the E-ring or the like. Therefore, for example, a saw chain such as a chain saw is driven around a portion of the crankshaft adjacent to the one half body, which means that the portion of the crankshaft adjacent to the one half body is subjected to a greater load than a portion of the crankshaft adjacent to the other half body.    Patent Document 1: JP-A-63-124863 (see FIG. 1)    Patent Document 2: JP-A-07-189704 (see FIG. 2)